EXTERNAL FORM OF EMBRYO OF 10 MM. 



223 



sac, and the allantois, both of which pass through the cord to join internal struc- 

 tures of the embryo. 



Pig Embryo of 10 mm. (Fig. 166). The form of the embryo has undergone 

 notable changes, as comparison with figure 165 will show. The head is larger, the 

 expansion in the regions of the mid- and fore-brains being particularly noticeable. 

 The limb-buds have lengthened, as has also the umbilical cord. The third branchial 

 arch has disappeared from the surface into the cervical sinus. The head as a 

 whole lies nearly at right angles with the back, so that the dorsal outline of the 



Yen. Md. Au. 



C.S. 



A.L. 



Um 



M.L. 



P.L 



FIG. 166. PIG EMBRYO OF 10 MM. 



A.L, Anterior limb. Au, Auditory, or first gill-cleft. C.S, Cervical sinus. Md, Mandibular process. M.L, 

 Milk-line. MX, Maxillary process. N, Nasal pit. Op, Eye. P.L, Posterior limb. Seg, Muscular seg- 

 ment. Um, Umbilical cord. Ven, Floor of fourth ventricle (medulla oblongata). X 8 diams. 



head forms a distinct though rounded angle with that of the back. This angle 

 marks the position of the neck-bend, and also the junction of the brain with the 

 spinal cord. The very distinct neck-bend is characteristic of the mammalian embryo. 

 It is less evident in birds and reptiles, absent in amphibians and fishes. Its devel- 

 opment probably causes the cramping of the ventral cervical region, which leads 

 to the formation of the cervical sinus, C.S, and to the disappearance from the 

 surface of the second, third, and fourth gill-clefts. Another consequence of the 

 neck-bend is the approximation of the nasal regions, N, of the head to the cardiac 

 region of the body. The cephalic region has a second flexure, the head-bend 



